The hormone secreted by the thyroid gland is:

Each endocrine gland in the body secretes its own characteristic hormone(s) directly into the bloodstream, and each hormone acts on specific target tissues; identifying "which hormone comes from which gland" is a matter of matching the gland to its known secretion. The thyroid gland, located in the front of the neck, secretes the iodine-containing hormones thyroxine (T4) and triiodothyronine (T3), which together regulate the body's basal metabolic rate, growth, and development.

Applying this to the given options: thyroxine is exactly the hormone produced by the thyroid gland, so it fits the question directly.

Checking the other options confirms this by contrast:

• Insulin is secreted by the beta cells of the pancreas to regulate blood glucose, not by the thyroid.

• Parathormone is secreted by the parathyroid glands to regulate blood calcium and phosphate, not by the thyroid.

• "None of the above" would only be correct if no listed hormone came from the thyroid, which is not the case here.

Since thyroxine is the only listed hormone that the thyroid gland itself secretes, it is the correct answer.